Tracking down Interference
Tracking down interference in the 2m amateur band

Whether it’s an annoying buzz or hum, radio interference can certainly diminish enjoyment of our hobby. Finding it and getting it fixed can be one of those challenges that can really try us, but it need not be. This is the story of my local QRM and how I fixed it.


Every so often, a loud hum would open the squelch on my 2 metre FM radio. This would happen at all hours of the day and night, and after several months, it wore a bit thin. The interference or QRM sounded like a 50Hz hum that seemed to increase and decrease in amplitude. It came and went over many months to the point where I’d almost forgotten about it. and then, when i least expected it, it was back with a vengeance. It had taken what seemed to be 5 or 6 months before I decided to get off my backside and do something about it.

In a previous life, I’d been a keen foxhunter, that’s amateur radio radio direction finding before you start emailing. At least two decades of my post-uni, amateur radio life had been spent chasing radio transmitters through the parks and gardens of Melbourne and beyond. I’d built many ‘sniffers’, antennas, pre-amps and the odd fox (transmitter). I’ve even made the pilgrimage to South Australia for the National Championships a few times, and even got the sport on TV, remember Clive Robertsons World Tonight? That’s a story for another time. I digress….


One of the best things for monitoring interference is a SDR radio. Sofware Defined Radios are usually computer controlled radios which are controlled by 3rd party party software. I have a RSP1 which is controlled by SDRuno What you’ll see on this page is videos and screen grabs of the software in action. I also have an IC-9700 which is also a new generation SDR from Icom. It is connected to a rotatable 2m yagi. Finally, I have a few two metre sniffers, these are 3 element, 2m yagis with a receiver that converts the strength of the radio signal to an audio frequency. The stronger the signal, the higher the tone. For this exercise I used a VK3YNG sniffer.

The task at hand

It was Saturday 28th of September. The QRM had just broken the squelch on the FM box so I decided to swing the beam around and see if I could get a direcion on the interference. Initially I thought it may be local, so after a few quick tests, like turning off the network router and modem etc, it didn’t go away. The signal on the 9700 was very strong, and very wide. From my QTH I had a good northerly bearing.

Tracking down interference can be one of the most challenging tasks in the hobby of amateur radio. Some sources of interference are constant, others vary. Sometimes interference can shift frequency, like in this case, and some are rock solid. Some cases can even be right under your nose or many miles away. Some can even be temporary – at a roadworks site or construction area.

The SDR (pictured) shows the interference I was experiencing. Click the image to enlarge it and you’ll see the CW beacon in Waverley, some 50km away, some local LAN noise, and the offending QRM. The waterfall shows it drifting all over the band.

The interference was heard as low as 145.0MHz and as high as 147.5MHz

Tracking down VHF Interference on the 2m band
Interference on the IC-9700 waterfall

The image above shows the drift of the interference on the IC-9700. Note the signal strength with an attenuator in.

SDRuno interference width

This image shows the width and drift of the signal on the SDR reciever.

Having determined the source of the interference was not local, I grabbed a sniffer out of the garage and swung it around to see if the direction of the inteference matched that of the yagi on the 9700. It did – a heading of 5ø appoximately. So it was time to scare the neighbours and go for a walk down the street. What could it be? Someone’s cheap LED downlights? A solar inverter maybe?

I’d walked about 1km and soon realised this signal wasn’t as local as I thought, so i walk back home, grabbed a hand held, and jumped in the car. After a quick look at Google Maps, I drove about 2 km north of my QTH monitoring the signal all the time on the dual bander in the car. The signal was getting very strong near a local primary school. Hmmm, a security monitor maybe?

It was time for a stroll with the sniffer through the school.

Remembering just how much fun fox hunting was, I headed off into the primary school armed with a sniffer and a handheld. As the frequency drifted I could follow it on the handheld, and then dial it into the YNG sniffer. At home, the QRM was range 1, a measure of attenuation. Not very much. In the school, the QRM was range 4 to 5, which is extremely close! 5 is as strong as it gets with a fully saturating signal. It was dark, and as I emerged at a gate on the other side of a playground, I was in a street with only a few residential houses on relatively large blocks.

I could go no further. A wall of fenced houses and gated driveways meant I had to get to the other side of the houses to confirm the souce, so back to the car I went.

Now back in the car, the interference was full scale on both handheld and mobile.

A quick look at Google Maps showed there was no road behind these properties as I was in a semi rural area. The best I could to was either side. Having driven to a few streets at right angles to the source, the sniffer was pointing back to one house in particular. At this stage, I figured I’d nailed it. I kept going back to the same two story house which was very new. It still had the builder’s sign up out the front. So that was the Saturday night sucess. Tomorrow, I’d come back in the daylight to confirm  my suspicions.

A quick return visit on Sunday 29th confirmed the source of the interference. So it was time to call in the ACMA to do the approriate door knocking.

Interference strength In the
Out with the sniffer

The ACMA has a form on their website to lodge a request to investigate interference.  On Sunday arvo I filled out the form and received an email confirmation. ACMA want as much information as you can give them. Like most organisations they’re resource-strapped. Here’s a snippet from the acknowledgement email showing the type of information supplied.

ACMA interference email response

The following Tuesday, October 1, I received a call from the local Radio Insepctor. He said he was in the area the following morning and would take a look. There was a brief Q&A session just to get a little more information. On my arrival home after work on October 2, I was pleasantly surprised to find the QRM was gone. The band was free of nasty hum and annoying squelch opening buzzes. I was suitably impressed that notifying ACMA on Sunday resulted in a solution by Wednesday. Just 4 days!

A few days later I received a follow-up phone call from ACMA. The culprit was a cheap masthead TV amplifier that coupled itself into the TV antenna which was pointing north to the Mount Dandenong TV transmitters straight up Peninsula Link. This explained why the QRM could be heard 10 to 15 km away. The owner was asked to replace it. I’m guessing they have.

Some helpful hints

  • Collect as much information over time as possible about the interference including time off day heard, intermittent or continuous, variation in strength, and importantly the weather. Also any characteristics of the interference such as any audio that maybe detected.
  • Make an audio recording of the interference if possible as the ACMA representitive asked to hear the QRM over the phone.
  • If you have a SDR program take some screen grabs of the QRM. Better still, screen record it with audio. I uploaded several grabs when reporting the interference.
  • If you have directional antennas, plot the heading on a map. narrowing down the source is very advantages. A street address or Lat Long is much appreciated by those investigating.
  • If you find the signal gets too strong as you get closer, try de-tuning your receiver or swapping your efficient antenna for something less efficient – a handheld antenna or even a dummy load.
  • Let the ACMA do the door knocking.

Further Reading

ACMA Interference to radio communications

QRM guru is an online educational and general information resource by RASA.